The present invention relates to an electric power tool that includes a direct current motor as a power source and controls a rotation of the direct current motor depending on an operation amount of a trigger switch.
A conventional electric power tool of this type includes a bridge circuit (in other words, an inverter circuit) which is constituted by switching devices (high-side switches and low-side switches), each provided between each of terminals of the direct current motor and a positive electrode and a negative electrode of a direct current power source (a battery), as a drive circuit for the direct current motor.
When a user operates a trigger switch, a control circuit calculates, on an operation amount, a driving duty ratio for PWM controlling the direct current motor and turns on/off the switching devices in the bridge circuit in accordance with the driving duty ratio, thereby controlling a rotation of the direct current motor.
In the electric power tool, when the direct current motor as a power source is mechanically locked during driving, a large electric current may flow through the direct current motor or the drive circuit therefor, thereby causing these components to be heated and burnt out in some cases.
Accordingly, the electric power tool usually includes a protection circuit. The protection circuit determines whether or not a current flowing through the direct current motor (i.e., a motor current) has reached a predetermined current value for lock determination during driving of the direct current motor, thereby determining whether or not the direct current motor is locked. When it is determined that the direct current motor is locked, the protection circuit stops the driving of the direct current motor.